It is known to fit pacemakers to patients suffering from chronic heart failure where the walls of the ventricles (the main pumping chambers of the human heart) are no longer synchronised. Such pacemakers are known as biventricular (or resynchronising) pacemakers and can be used to effect cardiac resynchronisation therapy (CRT). A biventricular pacemaker stimulates both the left and right sides of the heart in order to shorten atrioventricular delay and improve synchrony of ventricular contractions but does not necessarily vary the heart rate per se. The fitting of a biventricular pacemaker to an appropriate patient has been observed to result in a prompt improvement in haemodynamic status, with an increase in peak rise in intraventricular pressure, an increase in stroke volume and consequently higher systemic arterial blood pressure.
In principle, there are, in fact, two classes of biventricular pacemaker. An atriobiventricular pacemaker has the following three basic attributes, whose settings may be adjusted.
1) The heart rate. In many patients with resynchronizing pacemakers, the patient's natural heart rate is satisfactory, and the pacemaker is programmed merely to follow the natural heart rate. In other patients, the natural heart rate is too low, and the pacemaker is programmed to pace at a higher rate. In both groups of patients, the pacemaker may change between following the natural heart rate and actively controlling the heart rate, for example when the patient undertakes physical exertion.
2) The atrioventricular (“AV”) delay. This is the time interval between the atrium and the ventricles getting electrical stimulation. This delay is often set at about 120 ms when the pacemaker is initially implanted.
3) The left ventricle versus right ventricle (“LV-RV” or simply “VV”) delay. This is the time interval between the left ventricle and the right ventricle getting electrical stimulation. It is often set at 0 ms when the pacemaker is manufactured. Some manufacturers have a small non-zero lower limit, such as 4 ms, which can be treated as 0 ms for practical purposes.
The other class of biventricular pacemaker has two ventricular leads but, unlike the atrioventricular pacemaker, only the setting of VV delay can be adjusted and not the setting of AV delay.
In order to provide the optimum settings for these attributes, and especially atrioventricular (AV) delay, in a particular patient, many centres use an echocardiographic approach to selecting pacemaker programming. The most commonly used method is to determine, at resting heart rate, the longest filling time associated with complete atrial systole uninterrupted by ventricular systole. However, one problem with this approach is that there is little data to suggest that this approach optimizes hemodynamics in patients with chronic heart failure who have resynchronizing pacemakers.
It has been observed that blood pressure rises with the onset of biventricular pacing, and therefore it is theoretically possible to optimize the activity of a biventricular pacemaker by adjusting the attributes of the pacemaker while measuring the blood pressure of the patient. The problem with using a regular sphygmomanometer with an arm band cuff in order to measure blood pressure in these situations would be that taking each blood pressure measurement requires a considerable amount of time and, in practice, many different measurements would have to be taken during the optimisation process. Thus optimization by this method is entirely impractical.
It has also been proposed to determine blood pressure while optimizing biventricular pacemaker attributes by invasive haemodynamic monitoring of the blood pressure of the patient. However, the problem with this approach is that the clinical intricacy involved and the non-trivial risk associated with invasive blood pressure monitoring make it unsuitable for routine optimization of pacemaker attribute settings in normal practice.
WO2006/008535 discloses a method of optimising pacemaker settings by using non-invasive means, such as a finapres, to determine the blood pressure of the patient fitted with the pacemaker. WO2006/008535 also discloses that it is preferable to determine the optimum settings of a pacemaker while the patient has a raised heart rate. This is because patients with heart failure usually become symptomatic during exercise.
One problem with such prior art approaches to optimizing the settings of pacemaker attributes is that they require the heart rate to be raised by a patient exercising during the optimization process. Optimization during exercise has proven to be technically difficult using the currently available methods. Invasive haemodynamics are not suitable because of their invasive nature and echocardiographic techniques have limited use because of the practical limitation of acquiring images during exercise. Even if non-invasive means, such as a finapres, are used to determine haemodynamics, problems still occur for patients who are unable to exercise in order to raise their heart rate. Problems also occur at centres where the facilities are not available for exercise optimisation.